The invention relates to a data bus for vehicles having several safety related devices which constitute "subscribers" of the data bus.
Such devices include actuators such as airbags and belt tighteners, or also sensors which provide information regarding safety conditions and vehicle measurements. The data bus according to the invention differs in significance and function from known data buses which are common in motor vehicles, and which permit data exchange among all subscribers. Such subscribers include vehicle subsystems such as, for example, engine electronics, electronic hydraulic transmission control and an antilock system, which operate satisfactorily only if there is an exchange of data between all subscribers.
In a data bus for safety systems, in addition to the transit time and the time intervals between the recognition of a situation critical to safety and the reaction thereto (for example, the activation of an airbag), the main problem is to permit at least those subscribers which are important in a critical safety situation to have rapid access to the data bus. International patent document WO 89/09146 A discloses a controlled access method which requires hierarchical control for subscriber access to the data bus. Each subscriber has a time window which, according to its hierarchy, precedes or follows the subscribers arriving after or ahead of it. Each subscriber must wait until the time window of hierarchically preceding subscribers has expired, regardless of whether these subscribers are actually transmitting. In safety-critical systems, this requirement signifies a possibly unacceptable loss of time. On the one hand, the access of all subscribers to the data bus must be assured, and on the other hand, each subscriber must wait until the time windows of preceding subscribers have expired.
Alternative random access methods for the application as mentioned are also problematic, since several subscribers can transmit simultaneously (i.e., within the duration of the signal transit time), and therefore interference can occur. To avoid this problem, it is known to provide special systems for recognizing potential interference. However, the detectors used for this purpose increase costs and in the case of an interference require the establishment of retry time intervals, which likewise involves time delays. Hence the danger that a safety device (such as an airbag, for example) will not be deployed quickly enough when needed.
It is an object of the invention to provide a device which permits a plurality of subscribers to rapidly access a data bus which is connected to systems critical for ensuring passenger safety, without any interference.
This and other objects and advantages are achieved by the data bus according to the invention, in which each subscriber has a hierarchical transmission right, and each subscriber transmits only during an alarm situation. The right to transmit is obtained only at a point in time which is measured from an initialization point, and depends on its position within the hierarchy as well as the duration of transmission of the subscribers preceding it in the hierarchy. The time interval between two successive initialization points in time is selected such that the subscribers which must transmit during a critical condition can all and completely access the bus, while other subscribers can access the bus as transmitters only in the absence of a condition which is critical to passenger safety.
Each subscriber has a defined point in time for the beginning of its right to transmit. This depends, on the one hand, upon its hierarchical position, and on the other hand upon whether one or more hierarchically preceding subscribers are transmitting. If none of the priority subscribers is transmitting, it receives its right to transmit at the earliest possible point in time. If subscribers having a higher priority are transmitting, the point in time of its right to transmit shifts according to the number and length of the messages emanating from these subscribers.
The transmitting operation of the subscribers can be controlled via a cycling pulse which serves as an initialization pulse. By setting the cycling frequency (i.e., the frequency with which this pulse is emitted), the maximum time which passes from the occurrence of a critical safety situation until the particular subscriber accesses the data bus can be established. When necessitated, in order to activate (as quickly as possible) devices which are critical to passenger safety, it may be advantageous to shorten the cycling time (at that instant or with respect to the maximum time necessary for the emission of messages from all subscribers). As a result, it is no longer possible for all subscribers to access the data bus. Instead, it is advantageous to limit the number of subscribers to those which (in a situation critical to passenger safety) must absolutely have access to the data bus, while the other subscribers can in that case no longer access the data bus.
Simultaneously, or in addition, the shortening of the cycling time (in the manner described) can also raise the problem that a subscriber may still be transmitting after the end of the cycling time. Here, it may be advantageous to lengthen the cycling time accordingly and synchronize the following transmission of the initialization pulse with the second-to-last initialization pulse.
By means of the initialization pulse, it is possible to distinguish a critical state (alarm case) from a noncritical normal state (normal case). For this purpose, it can be advantageous to provide different lengths of the initialization pulse for the two conditions. The subscribers will then be shifted by the initialization pulse (more or less) to the alarm state.
The method according to the invention can be used to special advantage with a data bus based on a light waveguide. Such data buses require that the transmission operation of the subscribers be performed with high power, in order to compensate for the high power losses occurring in light waveguides, and to enable them to receive the transmitted message free from disturbances (in the case of subscribers which are at remote locations of the data bus).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.